UploadField was relying entirely on the File::get_class_for_file_extension to
select a class, so it could only create File or Image objects. This
would break the relationships based on derived objects. Also make it
respect the FileField::relationAutoSetting.
DataObject::get_one returns false if not found, so better check for
object. Also, the directory would not be cleaned, so on the subsequent
run the files could end up having suffixes.
missed this one
